A flowmeter and a method for determining the velocity of a fluid containing a known tracer substance having a known diffusion constant in the fluid are described in U.S. patent application Ser. No. 729,076, filed Nov. 4, 1976, now U.S. Pat. No. 4,107,525 in the name of Howard R. Hart, Jr., and assigned to the assignee of the instant invention. In the aforementioned Hart patent a fixed flux of tracer substance is introduced into the fluid in a conduit and concentrations of the tracer substance are measured as a function of position along the conduit by detectors, the tracer substance itself never leaving the confines of the conduit.